Apple iPhone 3GS

Time to upgrade. Wanting to buy the 3gs, but I don't want to change my data plan. Any sugestions?

As of right now, I am currently using a Pantech Matrix Pro. Before that, I was using a Blackjack II. My current data plan is unlimited messaging and unlimited internet usage. Can I carry this over to the 3gs? I don't want to shell out the $30 "Apple" plan. Do any iPhone users out there have any advice or stories that could help me?